GB2065961A

GB2065961A - Semiconductor laser

Info

Publication number: GB2065961A
Application number: GB8039500A
Authority: GB
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1979-12-13
Filing date: 1980-12-10
Publication date: 1981-07-01
Also published as: ES497606A0; US4375686A; IT8026552A0; GB2065961B; IT1134661B; ES8201774A1; JPS5694683A; FR2471683B1; DE3046238C2; DE3046238A1; JPS5932910B2; FR2471683A1; NL7908969A; AU6523380A

Description

1 GB 2 065 961 A 1

SPECIFICATION Semiconductor Laser

The invention relates to a semiconductor laser having a semiconductor body comprising a strip shaped active region situated within a resonator, in which laser radiation is emitted in the longitudinal direction of the strip-shaped active region.

Semiconductor lasers having a strip-shaped active region are frequently used nowadays, usually in the form of lasers having a single or a double heterojunction (DH lasers) as described, for example in Philips Technical Review, Vol. 36, No. 7, 1976, pages 190 to 200. These lasers comprise an active layer which on at least one side adjoins a passive layer having a larger band spacing, in which active layer the radiation is generated.

In these lasers, ageing phenomena occur in the course of their lives. For example, there may be an 85 increase in the threshold current, that is to say, the minimum current strength at which stimulated radiation emission occurs, and in the occurrence of spontaneous oscillations and pulsations, respectively, in the intensity of the 90 emitted radiation.

It has been found that the occurrence of such spontaneous oscillations is often associated with the formation, in the strip-shaped active region of the active layer, of regions having locally increased non-radiating recombinations of charge carriers and increased damping resulting therefrom. As a result of the increased non radiating recombination a local rise in temperature occurs so that the band spacing is slightly reduced. This in turn again leads to larger damping. These locally occurring regions often prove to be agglomerations of crystal lattice defects which have the tendency to propagate during the life of the laser over the whole strip shaped active region so that the laser eventually becomes useless. It has been found that lasers having a comparatively small active volume are less troubled by the occurrence of spontaneous oscillations. However, these lasers are often 110 difficult to manufacture.

In United States Patent Specification 3,510,795 a semiconductor laser is described having a strip-shaped active region into which charge carriers are injected by a plurality of mutually parallel arranged p-n junctions so as to obtain a better cooling.

According to the invention, a semiconductor laser having a semiconductor body comprising a strip-shaped active region situated within a resonator, wherein a number of parallel arranged p-n junctions are present to inject charge carriers into the active region and, during operation of the laser, radiation is emitted in the longitudinal direction of the strip-shaped active region, is characterized in that the strip-shaped active region is provided in an active semiconductor layer of a first conductivity type and comprises a plurality of zones of the second conductivity type within which radiation is generated which zones are connected electrically to an electrode provided on a surface of the semiconductor body, in that the part of the active layer between the zones is of the first conductivity type and has a band spacing which is at least equal to that of the zones, and in that, viewed in the longitudinal direction of the strip-shaped active region, the largest dimension of the zones is at most equal to micrometres.

Since the part of the active layer of the first conductivity type situated between the zones of the second conductivity type within which the radiation recombination occurs has a band spacing which is at least equal to that of the said zones, it is transparent to the generated radiation. Since the above- mentioned crystal lattice defects propagate under the influence of non- radiating recombination of charge carriers, this propagation of the lattice defects does not occur in the intermediate parts of the active region since substantially no recombination takes place there, and any lattice defects are restricted to the zones of the second conductivity type. In this manner no large continuous region with lattice defects can be formed in the active volume of the laser, so that spontaneous oscillations are avoided.

A further advantage of a semiconductor laser in accordance with the invention is that the first and higher order transverse modes of the radiation emanating from the zones of the second conductivity type have a direction of emanation which is inclined to the longitudinal direction of the strip-shaped active region. These modes of radiation thus tend to leave the active region before they can enter a subsequent zone to be further amplified. As a result of this, only amplification of a fundamental mode C'zeW' order) occurs and discontinuities in the curve of the radiation intensity as a function of the current strength are avoided. For this purpose, however, the zones of the second conductivity type should not be situated too closely together. Therefore the spacing of these zones is preferably substantially equal to or greater than their largest dimension in the longitudinal direction of the active region. It is to be noted that where in this application a dimension of a zone of the second conductivity type is mentioned, this is meant to be a dimension in a direction parallel to the active layer and the surface, respectively.

The spacing of the zones of the second conductivity type should not be so small that the charge carriers of the second conductivity type injected from a zone in the intermediate material of the first conductivity type of the active layer can move into the adjacent zone of the second conductivity type and recombine there.

In order to restrict the size of the continuous regions which can be occupied by grown lattice defects as much as possible, the largest dimension of the zones of the second conductivity type in the longitudinal direction of the active region is preferably at most 10 micrometres.

2 GB 2 065 961 A 2 The said zones preferably form part of regions of the second conductivity type which extend from the surface of the semiconductor body into the active layer. Although the zones of the second conductivity type, if desired, can extend through only a part of the thickness of the active layer, they are advantageously provided through the whole thickness of the active layer so as to obtain an optimum efficiency. The zones preferably terminate in the immediate proximity of the junction between the active layer and an underlying adjoining passive layer.

The active layer preferably consists of nconductivity type gallium arsenide or gallium aluminium arsenide in which p-type conductive zones are formed by doping with zinc, for example, by diffusion. Since in this case the ntype material has a large band spacing than the ptype material, the condition for the band spacing is automatically satisfied.

Finally it is of importance to note that, where the resonator is formed by reflecting side faces of the semiconductor body, for example, formed by cleavage faces of the crystal, it is of advantage when said reflecting faces intersect only the first conductivity type part of the active layer, which part is transparent to the radiation. In this case no recombination takes place near the reflecting faces so that they are less easily damaged. Thus the permitted radiation density of the laser can be increased by approximately a factor 10 relative to the case where the reflection faces intersect the material of the second conductivity type.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 is a plan view of a semiconductor laser in accordance with the invention, Figure 2 is a cross-sectional view of the laser shown in Figure 1 taken on the line 11-11, Figure 3 is a cross-sectional view of the laser shown in Figure 1 taken on the line and Figure 4 is a plan view of another embodiment of a semiconductor laser in accordance with the invention, Figure 5 is a cross-sectional view in the direction 111-111 of Figure 1 of a further embodiment of a laser in accordance with the invention, and Figures 6, 7 and 8 are plan views of other 115 embodiments of lasers in accordance with the invention.

The Figures are diagrammatic and not drawn to scale. For clarity, the dimensions in the direction of thickness are particularly exaggerated. In the cross- sectional views semiconductor regions of the same conductivity type are shaded in the same direction. Corresponding parts are generally referred to by the same reference numerals in the Figures. Figure 1 is a diagrammatic plan view and Figures 2 and 3 are diagrammatic cross-sectional views taken on the lines 11-11 and 111-111 of Figure 1 of a semiconductor laser in accordance with the invention.

The semiconductor laser has a semiconductor body 1 with a strip-shaped active region which is situated within a resonator and the boundaries of which are denoted in the plan view of Figure 1 by a and a'. During operation laser radiation, denoted in Figures 1 and 2 by the arrow 2, is emitted in the longitudinal direction of the strip-shaped active region. In this example the resonator is formed by two reflecting side faces 3 and 4 of the semiconductor body extending substantially perpendicularly to the active region, which side faces, in this case, are faces of the semiconductor crystal. A plurality of p-n junctions 5 is present to inject charge carriers into the active region. These p-n junctions are formed between the regions 6 and the underlying adjoining part of the semiconductor body to be described hereinafter.

In accordance with the invention the stripshaped active region is provided in the active semiconductor layer 7 of the first conductivity type. The strip-shaped active region and consequently the layer 7 comprises a plurality of zones 8 of the second conductivity type within which the radiation is generated. Via the above- mentioned regions 6, the zones 8 are connected electrically to an electrode 10 which is provided on a surface 9 of the semiconductor body and which in this case is formed by a metal layer. Furthermore, the part of the active layer 7 situated between the zones 8 is of the first conductivity type and has a band spacing which is at least equal to that of the zones'8. The largest dimension of the zones 8, viewed in the longitudinal direction of the strip-shaped active region, is at most equal to (in this example smaller than) 20 micrometres.

In this example the semiconductor body comprises a substrate 11 of n-type conductive gallium arsenide having a thickness of 100 micrometres and a doping concentration of 1018 silicon atoms per cm3. Provided thereon is a layer 12 of n-type conductive gallium aluminium arsenide of the composition Ga,,,Al.,,As having a thickness of 2 micrometres and a doping concentration of 5.1011 tin atoms per CM3. Present on this layer 12, hereinafter referred to as the first passive layer, is the already mentioned active layer 7 comprising n-type gallium aluminium arsenide having the composition Ga...I.Al....As and having a thickness of 0.2 micrometre, which layer has a doping concentration of 10111 tin atoms per CM3. Provided thereon is an n-type conductive second passive layer 13 of gallium aluminium arsenide having the composition Ga,.,,AI,.3.As, a thickness of 1.5 micrometres and a doping concentration of 5.10 tin atoms per cml. Present thereon is an n-type contact layer 14 of gallium arsenide having a thickness of 0.3 micrometre and a doping concentration of 5.1011 tin atoms per CM3. The ptype regions 6 comprising the zones 8 are obtained by diffusion of zinc through the windows 15 in an electrically insulating layer 16, for example of silicon oxide, aluminium oxide, silicon nitride or another dielectric material, provided on 3 GB 2 065 961 A 3 the surface 9 of the semiconductor body. On the lower surface of the substrate 11 a metal layer 17 is provided. The p-n junctions 5 can be biased in the forward direction by applying to the metal layer 10 a voltage which is positive relative to the metal layer 17. Laser action occurs at a current strength in the forward direction above the threshold current.

In known laser diodes of the double heterojunction type of the kind described the 75 diffused p-type region 6 would extend as a continuous strip-shaped zone over the whole length of the laser. Crystal lattice defects which are present in that zone give rise to non-radiating recombinations. During operation, in particular during continuous operation of such a laser, these crystal lattice defects propagate under the influence of non-radiating recombinations until the laser starts showing spontaneous oscillations in addition to an increased threshold voltage and eventually becomes useless. The crystal defects can propagate until they cover the whole p-type region.

In the present laser any crystal lattice defect present in any of the zones 8 can at most expand to the volume of this one zone. The intermediate regions of the active layer of the first conductivity type are as a matter of fact transparent to the radiation so no electron-hole pairs are formed therein and hence no recombination occurs there either. The zones 8 which comprise no crystal defects are therefore not affected by the propagation of crystal defects in other zones.

It has been found that by using a laser structure in accordance with the invention spontaneous oscillations with the associated disadvantages can be avoided. Additional advantages of the laser described are: a better thermal cooling than in known DH lasers and absence of astigmatism due to the refractive index step in lateral direction between the zones 8 and the n-type material.

Although for clarity only five z ones 8 are shown in the drawing, this number will usually be much larger. In the semiconductor laser of the 110 example described, the number of said zones may be, for example, 25. The zones may have dimensions of approximately 6 x 6 micrometres; the spacing of the zones may be approximately 6 micrometres and the overall length of the laser may be 334 micrometres. The side faces 3 and 4, as shown in Figures 1 and 2, were provided so that they intersect only the n-type part of the layer 7 at a distance of approximately 20 micrometres from the. nearest zone 8. As a result of this 120 substantially no recombination occurs at the reflection faces 3 and 4 and the radiation intensity at which irreparable damage of said side faces occurs is approximately 10 times as high as when the reflection faces 3 and 4 intersect the p-type regions 6.

The semiconductor laser described above can be manufactured, for example, as follows. The starting material is an n-type substrate 11 of GaAs having a doping concentration of 1011 silicon atoms per CM3. Grown successively thereon by liquid phase epitaxy are the layers 12, 7, 13 and 14 having the above-mentioned thicknesses and doping concentrations. Growth from the liquid phase is a generally used method in semiconductor technology and the details thereof need not be entered into. Reference is made to pages 433 to 467 of the book entitled "Crystal Growth from High Temperature Solutions- by D. Elwell and J. J. Scheel and published by Academic Press, 1975.

A 0.15 micrometre thick masking layer 16, in this example of aluminium oxide (A1203) is then provided on the surface 9 of the layer 14. This may be done, for example, by vapour deposition. Windows 15 are etched in the layer 16 using conventional photoetching methods. Concentrated phosphoric acid (H3P04) at 701 may be used.

Zinc is then diffused through the windows 15 at 6201C, for example, in an evacuated capsule having ZnAs, as a diffusion source, the aluminium oxide layer 16 serving as a mask. The duration of the diffusion depends on the thickness of the layers 7, 13 and 14. With the above layer thicknesses (given by way of example) this is approximately 90 minutes. P-type regions 6 are then obtained which extend approximately down to the interface between layers 12 and 7.

Alternatively, prior to the diffusion of the zinc the layer 14 may be removed at the area of the windows 15 by means of a selective etchant which attacks GaAs but does not attack gallium aluminium arsenide. Such an etchant is composed, for example, of 25 cm' of H202 30% and 25 CM3 of water replenished with NH,OH to pH=8. In this case the diffusion need not be so deep so that lateral diffusion is reduced.

The metal layers 10 and 17 are then provided, the layer 10 being, for example, a chromium layer and the layer 17 being a gold-germanium-nickel alloy. Finally the laser is mounted on a cooling member in the usual manner, preferably with the metal layer 10, and provided in an envelope.

Instead of using liquid phase epitaxy, the above-described laser can be manufactured in a different manner, for example, by epitaxial growth from the gaseous phase. Instead of the semiconductor material used in this example, other semiconductor materials suitable for laser manufacture may also be used.

It is to be noted that, although in this example the reflection members are formed by cleavage faces of the semiconductor crystal, other reflection members may also be used. For example, they may be formed by providing in or near the active region, taken in the longitudinal direction of said region, a geometric periodic change in the index of refraction and/or the layer thicknesses. This is the structure of the distributed feed back lasers (DFB lasers) as described, for example, in Applied Physics Letters, Vol. 15, February 197 1, pages 152 to 154.

For illustration Figure 1 also shows diagrammatically the directions 2 in which, for 4 GB 2 065 961 A 4 example, laser radiation of the first order transverse mode generated in the zone 8 leaves this zone. When, as in this example, the spacing of the zones 8, taken in the longitudinal direction of the active region, is at least of the same order as the largest dimension of the zones 8, the greater part of the radiation in the directions 2 does not enter the adjacent zone 8 and is hence not further amplified. This applies to an even greater extent for the higher order modes. As the first and higher order transverse modes are more suppressed a slightly wider strip-shaped active region may in principle be used than is the case in known lasers having a continuous active zone.

Conversely, it may be said that a semiconductor laser in accordance with the invention laser radiation is generated with substantially only one transverse mode of oscillation.

As indicated in the example described (see Figure 1) the zones 8 of the second conductivity type viewed in the longitudinal direction of the active region are advantageously situated in a row with their centres on one straight line (11-1i). In certain circumstances, however, it may be advantageous that they are situated with their centres alternately on one of two parallel straight lines (see Figure 4), the distance between these parallel lines being preferably smaller than the largest dimension of the zones in the direction of the width of the active region. This is of advantage in particular when the zones 8' which are situated on one parallel line IV-11' are connected to a first electrode 10', and the zones 8" which are situated on the other parallel line il"-11" are connected to a second electrode 10", as is shown diagrammatically in the plan view of Figure 4. According as one or the other of the electrodes 10' or 10" is energised relative to the electrode 17, a laser beam 2' or 2'1 will be generated in which the distance between the emanating beams may be very small, and considerably smaller than is possible in known lasers for generating a -movable- laser beam.

The invention is not restricted to the examples described. For example, it is not strictly necessary that in the embodiment of Figures 1, 2 and 3 the p-type regions 6 extend through the whole thickness of the active layer 7, but it is sufficient in particular when the zones 8 extend through only a part of the thickness of the layer 7, as shown diagrammatically in the cross-sectional iew of Figure 5. A deeper zinc diffusion extending into the layer 12, may also be used.

Furthermore the laser need not be a double heterojunction laser. A single heterojunction laser in accordance with the invention may have the first passive layer 12 and the active layer 7 (see Figure 1) made of a semiconductor material of the same composition.

The dimensions of the zones of the second conductivity type need not all be equal. In particular, according to the longitudinal direction of the active region. the transverse dimension of the zones may have a periodic variation. This is shown didgrammatically in the plan view of Figure 6. Such a structure has properties analogous to those of the laser structure having periodic width variation of the strip-shaped active region as described in U.K Patent Specification

GB 2,033,647.

In addition to the semiconductor material of the various layers the doping materials for the zones 8 may also be varied. The various conductivity types may be replaced by their opposite types provided the condition for the band spacings in the active layer is met. For example, in certain circumstances instead of the zones 8 the material of the layer 7 between the zones 8 may be obtained by doping. This doping need not be effected by diffusion but may take place in certain circumstances by ion implantation or in some other way.

Finally it is to be noted that although in the examples the zones of the second conductivity type are square or rectangular in shape, in practice this need not always be the case and zones having circular, triangular or other shapes, as shown, for example in Figures 7 and 8 are also within the scope of the invention.

Claims

1. A semiconductor laser having a semiconductor body comprising a stripshaped active region situated within a resonator, wherein a number of parallel arranged p-n junctions are present to inject charge carriers into the active region and, during operation of the laser, radiation is emitted in the longitudinal direction of the stripshaped active region, characterized in that the strip-shaped active region is provided in an active semiconductor layer of a first conductivity type and comprises a plurality of zones of the second conductivity type within which radiation is generated, which zones are electrically connected to an electrode provided on a surface of the semiconductor body, in that the part of the active layer between the zones is of the first conductivity type and has a band spacing which is at least equal to that of the zones, and in that, viewed in the longitudinal direction of the strip-shaped active region, the largest dimension of the zones is at most equal to 20 micrometres.

2. A semiconductor laser as claimed in Claim 1, characterized in that the largest dimension of the zones of the second conductivity type in the longitudinal direction of the active region is at most 10 micrometres.

3. A semiconductor laser as claimed in Clairn 1 or 2, characterized in that the spacing of the zones of the second conductivity type in the longitudinal direction of the active region is substantially equal to or greater than their largest dimension.

4. A semiconductor laser as claimed in any of the preceding Claims, characterized in that the zones of the second conductivity type, viewed in the longitudinal direction of the active region, are situated with their centres on one straight line.

5. A semiconductor laser as claimed in any of the Claims 1 to 3, characterized in that the zones GB 2 065 961 A 5 of the second conductivity type, viewed in the longitudinal direction of the active region, are situated with their centres alternately on one of 30 two parallel straight lines.

6. A semiconductor laser as claimed in Claim 5, characterized in that the distance between the parallel lines is smaller than the largest dimension of the zones in the direction of width of the active region.

7. A semiconductor laser as claimed in Claim 5 or 6, characterized in that the zones which are situated on one parallel line are connected to a first electrode and the zones which are situated on the other parallel line are connected to a second electrode.

8. A semiconductor laser as claimed in any of the preceding Claims, characterized in that the zones of the second conductivity type all have the same dimensions at least in the longitudinal direction of the active region.

9. A semiconductor laser as claimed in any of the preceding Claims, characterized in that the zones of the second conductivity type in the active layer form part of regions of the second conductivity type extending from the surface of the semiconductor body at least into the active layer.

10. A semiconductor laser as claimed in any of the preceding Claims, characterized in that the zones of the second conductivity type extend through the whole thickness of the active layer.

11. A semiconductor laser as claimed in any of the preceding Claims, characterized in that the active layer is situated between two passive layers of the first conductivity type having a larger band spacing than that of the active layer.

12. A semiconductor laser as claimed in any of the preceding Claims, characterized in that the active layer consists of n-conductivity type gallium arsenide or gallium aluminium arsenide.

13. A semiconductor laser as claimed in Claim 12, characterized in that the zones of the second conductivity type are formed by doping with zinc.

14. A semiconductor laser as claimed in any of the preceding Claims, characterized in that the resonator is formed by reflecting side faces of the semiconductor body, said side faces intersecting only the first conductivity type part of the active layer. 50

15. A semiconductor laser substantially as herein described with reference to Figures 1 to 3 of the accompanying drawings.

16. A semiconductor laser substantially as herein described with reference to any of Figures 4 to 8 of the accompanying drawings.

Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1981. Published by the Patent Office, 25 Southampton Buildings, London, WC2A 1 AY, from which copies may be obtained.